This invention relates to a holographic shearing interference contrast method for the observation of a transparent object or phase object.
When a light passes through a phase object (hereinafter referred to merely as "object") which absorbs light, the light wave loses intensity. Since human eyes and photosensitive films are sensitive to such a variation in the light intensity, the image of such an object can be observed or recorded through an ordinary optical microscope. In the case of a perfectly transparent object, however, the image of the object cannot be observed through an ordinary optical microscope because the object does not absorb light. Whenever a light passes through any object, there is produced a lag in phase. To permit the observation of the object, therefore it becomes necessary for the phase difference of light to be converted into corresponding difference in intensity. One of the methods which are available for the purpose of this conversion is the phase contrast method which is employed such as in conventional phase contrast microscopes. According to this method, a diffraction beam which has passed through a given object and a beam which has advanced without passing through any obstacle are passed through a transparent object to give rise to a phase difference of .pi./2, namely, a contrast is given to the image of the article in proportion to the index of refraction of the transparent object.
The shearing interference method is also available for the aforementioned conversion. This method enables a transparent object to be observed in a shape made up of interference fringes. The shearing interference contrast method is the product of the combination of this shearing interference method and the aforementioned contrast method. Unlike the shearing interference method which resorts to observation of a shape made of several interference fringes rather difficult of discernment, the shearing interference contrast method enables the image of the object itself to be observed by virtue of variation in the light intensity instead of the shape made up of interference fringes. It is also capable of allowing observation of a slight variation in the index of refraction even smaller than the wavelength of light. This method obviates the necessity for a phase plate and other similar accessorial means.
The shearing interference contrast method accomplishes the desired observation of a given object by dividing the wavefront issuing from the object into two portions (amplitude division), shearing the divided wavefronts laterally, causing the two wavefronts to interfere with each other by giving them a phase difference equivalent to one fourth of the wavelength (.pi./2) and converting the variation in phase into a corresponding variation in light intensity. By this method, since the wavefront issuing from the object is divided into two portions and the phase between these two wavefronts is continuously varied, the image of this object can be observed in the form of the light and dark image obtained when a light diagonally impinges upon an uneven object. So far as a given object has its phase varied as described above or, in the absence of an uneven surface, has its index of refraction varied similarly, the image of the object can be observed in the form of light and shade so as to produce a three-dimensional illusion on the part of the observer.
Formerly the inventor suggested an interferometer which operates by the application of holograms to the shearing interference method ("Holographic shearing interferometer": U.S. Pat. No. 4,118,124, dated Oct. 3, 1978. The interferometer uses a set of two holograms each produced by the interference fringes due to two collimated planar beams having slightly different angles of incidence and one reference beam. These two holograms are set in position and the beam which has passed through a given object or has been reflected by the object is caused to impinge upon the first hologram as a reference light. Behind the first hologram, there are obtained two first order diffraction beams equivalent to the two collimated planar beams which have served the purpose of forming interference fringes on the first hologram. At this time, the two diffraction beams of the first hologram impinge at different positions on the object to cause object beams. This means that the diffraction beams are sheared by this difference in position. When the two object beams impinge upon the second hologram, they reproduce one first-order diffraction beam traveling in the same direction as the reference beam and give rise to interference fringes behind the second hologram. These are shearing interference fringes which occur when the wavefronts of a given object are laterally sheared by an amount to be determined by angles of incidence of the two collimated planar beams and the distance, between the first hologram and the second hologram and are consequently allowed to interfere with each other. The amount of this shearing can be adjusted by suitably varying the distance between the two holograms. The conventional holographic interference method necessitates use of one new hologram for each observation because the observation inevitably requires the information on the wavefront of the particular object under observation to be recorded on that hologram. The interferometer designed and operated by method permits the desired observation to be obtained on a virtually limitless number of objects because it incorporates a set of two holograms as a component part thereof and obviates the necessity for photographing the holograms at the time of each observation. Further, this method has an advantage that since the desired observation of an object which either passes or reflects beam can easily be obtained by merely having the object placed in the path of the reference beam, the interferometer is capable of providing effective observation even for large objects.
An object of this invention is to provide, through application of the holographic shearing interference method to the shearing interference contrast method, a shearing interference contrast method using holograms which, unlike the conventional contrast method which suffers from various disadvantages such as poor feasibility of manufacture, limited capacity of observation, mechanical instability and difficult adjustment, enjoys many advantages such as high resolving power, notable stability to withstand vibrations and other external impacts, wide field of observation, ease of adjustment and ability to provide heretofore unattainable quantitative measurement.
Another object of this invention is to provide an interferometer or interference microscope adapted for the holographic shearing interference contrast method mentioned above.